As the optical communication technology moves forward, development of optical components configured to subject an optical signal directly to signal processing becomes increasingly important. Above all, a waveguide-type optical interferometer utilizing interference of light and using a planar lightwave circuit (PLC) integrated on a planar substrate, has advantageous features including mass production, low cost and high reliability, and many pieces of research and development thereof have been carried out. The waveguide-type optical interferometers include an arrayed waveguide diffraction grating, a Mach-Zehnder interferometer, and a lattice circuit, for example.
Such waveguide-type optical interferometers are fabricated by use of standard photolithography and etching technologies and glass deposition technology such as FHD (flame hydrolysis deposition). Specifically, the outline of the process is as follows. First, an undercladding layer, and a core layer with a higher refractive index than its surroundings are deposited on top of a substrate. Then, a waveguide pattern is formed in the core layer. Finally, the waveguide formed in the core layer is buried by an overcladding layer. With such a process, the waveguide is fabricated. An optical signal propagates through the waveguide, while being confined within the waveguide fabricated through such a process as is mentioned above.
Now letting us turn our eyes to modulation/demodulation process technology for an optical transmission system, signal transmission using phase modulation method is in wide, practical use. Differential phase shift keying (DPSK), in particular, attracts attention in that it is strongly resistant to signal degradation caused by chromatic dispersion or polarization mode dispersion in a transmission line. Further, the implementation of multilevel modulation, i.e., an increase of the number of constellation points for phase modulation, also takes place. Research has been done also on DQPSK (differential quadrature phase shift keying) with four constellation points, or the like, in addition to DBPSK (differential binary phase shift keying) with two constellation points.
Demodulation of such a DBPSK or DQPSK optical signal requires an optical delay line interferometer configured to perform demodulation by causing interference between optical signals corresponding to contiguous symbols. Specifically, a phase difference between optical signals of contiguous symbols can be demodulated by dividing an optical signal into optical signals, delaying one optical signal by a time equivalent to one symbol, and causing the one optical signal to interfere with the other optical signal. Application of the PLC technology previously mentioned for fabrication of the optical delay line interferometer is expected to achieve a long period of stabilization of circuit performance, circuit miniaturization, or the like.
Conventional Art 1:
FIG. 1 is a diagram showing a basic configuration of an optical delay line interferometer that forms a DPSK demodulator. Operation of the optical delay line interferometer will be described below, taking DBPSK with two constellation points as an example. An optical delay line interferometer 1 is formed by one Mach-Zehnder interferometer. Specifically, the optical delay line interferometer 1 includes, in its input, a first input waveguide 2 and an optical splitter 3 connected to the input waveguide 2, and includes, in its output, first and second output waveguides 6 and 7 and an optical coupler 10 connected to the output waveguides. The optical splitter 3 and the optical coupler 10 are connected by two waveguides of different lengths, namely, a long arm waveguide 4 and a short arm waveguide 5. A pair of photodiodes (hereinafter called “PDs”) 8a and 8b that form a balanced photodetector are disposed at the output ends of the first and second output waveguides 6 and 7, respectively.
The optical delay line interferometer 1 receives a DBPSK optical signal coming in at the input waveguide 2. The DBPSK optical signal is divided into two optical signals, the long arm waveguide 4 and the short arm waveguide 5, by the optical splitter 3. The long arm waveguide 4 and the short arm waveguide form an optical delay line 9 by a difference in length between these waveguides. The amount of delay time provided by the optical delay line 9 is time equivalent to one symbol of the DBPSK optical signal. For example, if a symbol rate is 20 Gbaud, a delay of 50 ps, the reciprocal of the symbol rate, is the amount of delay time for one symbol. The optical delay line 9 gives this delay thereby to generate interference between the DBPSK optical signals across their contiguous symbols. The DBPSK optical signals subjected to the interference exit through the two output waveguides 6 and 7, respectively, to the balanced photodetector, which then in turn detects a light intensity difference between the optical signals. This results in generation of a demodulated signal corresponding to a phase difference between the contiguous symbols. In other words, the demodulated signal is obtained from the balanced photodetector formed of the pair of PDs 8a and 8b. For example, a phase difference of 0 provides a positive demodulated signal, and a phase difference of π provides a negative demodulated signal.
In the case of binary or further multileveled phase modulation, the demodulation of the phase difference is not possible with one Mach-Zehnder interferometer alone. For instance, a DQPSK demodulator with four levels requires two Mach-Zehnder interferometers for demodulation of a modulated optical signal. Description will now be given of a configuration of an optical delay line interferometer for DQPSK.
Conventional Art 2:
FIG. 2 is a block diagram showing a conventional optical delay line interferometer for DQPSK. Referring to Patent Document 1, the most basic optical delay line interferometer as employed in the conventional art has a configuration such that DQPSK optical signals predivided by an optical splitter are inputted to two different Mach-Zehnder interferometers, respectively. Specifically, one of DQPSK optical signals divided by an optical splitter 23 is inputted to a first optical delay line interferometer from an optical splitter 3a through a pair of waveguides 4a and 5a to the pair of PDs 8a and 8b. The other DQPSK optical signal is inputted to a second Mach-Zehnder interferometer from an optical splitter 3b through a pair of waveguides 4b and 5b to a pair of PDs 8c and 8d. The two Mach-Zehnder interferometers include optical delay lines 9a and 9b, respectively, which each provide a delay of one symbol. Further, the one optical delay line 9a includes a (π/2) phase shifter 12.
Variable phase adjusters are disposed in the optical delay lines 9a and 9b and the phase shifter 12 thereby to effect fine adjustment of the delay time or the amount of phase shift. For instance, if a heater is used to form the variable phase adjuster, heating the heater disposed in an upper portion of the waveguide can effect a change in refractive index and hence a change in optical path length. Thus, fine adjustment can be made on the delay time or the amount of phase shift to be given, by the amount of increase in temperature of the heater.
According to the above-mentioned configuration, the optical delay line interferometers demodulate separate orthogonal binary signals (i.e., an I signal and a Q signal), respectively, thereby enabling the demodulation of the DQPSK optical signal with four levels, taken as a whole. With the above-mentioned configuration, however, if the required number of Mach-Zehnder interferometers increases as the number of constellation points is increased for multilevel modulation, an area in the PLC, occupied by the optical delay line interferometer taken as a whole, multiplies, which in turn likewise leads to an increase in cost per chip. For the demodulation of a multileveled modulated signal, therefore, the utilization of the optical delay line interferometer of the configuration shown in FIG. 2 is undesirable.
Conventional Art 3:
FIG. 3 is a block diagram showing another conventional optical delay line interferometer for DQPSK. This configuration presents a proposal of a method in which one optical delay line is utilized to form substantially two Mach-Zehnder interferometers. Specifically, two DQPSK optical signals divided from a DQPSK optical signal by the optical splitter 3 are inputted to a pair of waveguides, namely, the long arm waveguide 4 and the short arm waveguide 5, respectively. The long arm waveguide 4 is provided with the optical delay line 9. Each of the optical signals from the waveguides 4 and 5 is further divided into two optical signals by optical splitters 13b and 13a, respectively. The two optical signals thus divided are recombined by two optical couplers 10a and 10b and thereby interfere with each other. Interfering lights from each of the optical couplers 10a and 10b are fed to the corresponding one of two balanced photodetectors 8a and 8b via output waveguides 6a and 7a, respectively, or output waveguides 6b and 7b, respectively (see Patent Document 1). The demodulation of the DQPSK optical signal can be accomplished by balanced detection of two optical signals exiting from each of the optical couplers 10a and 10b. 
Any of four waveguides that link the optical splitters 13a and 13b and the optical couplers 10a and 10b may be provided with the (π/2) phase shifter 12. The variable phase adjusters are disposed in the optical delay line 9 and the phase shifter 12, respectively, as is the case with the conventional art 2. Fine adjustment enables an accurate provision of a desired amount of phase shift for the optical delay line 9 and the phase shifter 12. According to the above-mentioned configuration of the optical delay line interferometer, one common optical delay line alone can form substantially two Mach-Zehnder interferometers. The use of the one common optical delay line for formation of the optical delay line interferometer enables miniaturization of the optical delay line interferometer taken as a whole, as compared to the configuration of the conventional art 2.
In the Mach-Zehnder interferometer that forms each of the above-mentioned optical delay line interferometers, a wavelength λ0 at which transmittance of the optical signal outputted from one of the two output waveguides is at its maximum is expressed by the following equation:λ0=n×ΔL/k  Equation (1)where n denotes an effective refractive index of the long arm waveguide; ΔL denotes an optical path length difference between the long arm waveguide and the short arm waveguide; and k denotes a natural number. When the wavelength of the optical signal inputted to the Mach-Zehnder interferometer is λ0, the transmittance of the optical signal outputted from one of the two output waveguides is at its maximum, while the transmittance of the optical signal outputted from the other output waveguide is at its minimum.
For the optical delay line interferometer to demodulate a phase-modulated optical signal, it is required that a wavelength λs of the optical signal coincide with an optimum operating wavelength λc of the optical delay line interferometer. As employed herein, the optimum operating wavelength λc refers to a wavelength at which the phase-modulated optical signal can be demodulated at the highest S/N (signal-to-noise) ratio. Specifically, for example, λc is such that λc=λ0 for a DBPSK optical signal, or is such that λc=λ0+nΔL/(4k×(k−1)) for a DQPSK optical signal.
Referring to FIG. 1, methods for effecting a coincidence of λs and λc include a method in which a variable phase adjuster for the effective refractive index is disposed in the long arm waveguide 4 or the short arm waveguide 5 thereby to adjust the effective refractive index. For instance, if a heater is used to form the variable phase adjuster, heating the heater disposed in an upper portion of the waveguide can effect an adjustment of the effective refractive index. However, a change in temperature of the optical delay line interferometer leads to a mismatch between the wavelengths λs and λc, even if adjustment is such that the wavelength λs of the optical signal coincides with the wavelength λc at a given temperature T1. In other words, there arises the following problem: a change from T1 to T2 in the temperature of the optical delay line interferometer leads to the mismatch between the wavelength λs of the optical signal and the wavelength λc, because the effective refractive index has temperature dependence.
A solution to this problem necessitate adoption of means for keeping the temperature of the optical delay line interferometer constant, using a Peltier device or the heater, or adoption of means for keeping the variable phase adjuster in operation at all times. Any of these means presents the problem of increasing power consumption by the optical delay line interferometer, because of having to drive the Peltier device or the heater. In other words, it is necessary to lessen the temperature dependence of interference characteristics of the Mach-Zehnder interferometer and thereby reduce electric power required to compensate for the temperature dependence.
In order to reduce the temperature dependence of the optical delay line interferometer formed of the Mach-Zehnder interferometer, there has heretofore been used a method that involves forming a groove by removing the cladding and the core in a portion of the arm waveguide, and filling the groove with a material (hereinafter referred to as a “temperature compensation material”) having a different coefficient of refractive index dependence on temperature from a coefficient of effective refractive index dependence on temperature of the waveguide. Thereby, a transmission wavelength λ0 can become temperature-independent. This method is disclosed in detail in Patent Document 2, for example.
Conventional Art 4:
FIG. 4A is a diagram showing an example of the configuration of the optical delay line interferometer that forms the DPSK demodulator, in which the transmission wavelength λ0 of the Mach-Zehnder interferometer is made temperature-independent. In the configuration of this conventional art, a groove 14 is formed extending across the long arm waveguide 4, and the groove 14 is filled with the temperature compensation material. A variable phase adjuster 15 for the effective refractive index is disposed in the short arm waveguide 5.
FIG. 4B is a view showing, in enlarged dimension, grooves for temperature compensation. In order to reduce a loss of propagating light that occurs in the groove 14, the groove 14 is divided into multiple grooves 14a to 14e each having a width w, which are arranged at predetermined spaced intervals p. Conditions required for the temperature compensation material to be filled into the groove 14 are as follows. Specifically, it is particularly desirable that the temperature compensation material be such that the coefficient dn′/dT of refractive index dependence on temperature of the temperature compensation material is different in sign from the coefficient dn/dT of effective refractive index on temperature of the long arm waveguide, and |dn′/dT| is sufficiently larger than |dn/dT|. The temperature compensation materials that satisfy such conditions include a silicone resin for example, which satisfies the relationship (dn′/dT)˜−40×(dn/dT). Meanwhile, if the optical delay line interferometer is formed of multiple Mach-Zehnder interferometers, the transmission wavelength λ0 can be made temperature-independent by forming the same grooves in portions of the long arm waveguides 4 of the Mach-Zehnder interferometers, and filling the grooves with the temperature compensation material.
[Patent Document 1]: International Patent Publication No. WO2003/063515
[Patent Document 2]: International Patent Publication No. WO98/36299
[Patent Document 3]: Japanese Patent No. 2614365